Bridge Networking Device

Bridge

Bridge is a network device which divides a big network into smaller sub networks called segments. These segments are not consider as a separate network. Rather two segments can acts as a part of a single network because of bridge.

Bridges operates at both layers of OSI reference model that are physical layer as well as data link layer. Bridge is intelligence device which takes decision whether to forward the incoming frame on other port or discard it. It can also learn physical address of connected hosts and maintain this record in table called as forwarding table.

A bridge is basically a simplified two-port switch connecting two network segments. It does not have VLAN functions.

When bridge receives traffic on its any port, it checks incoming traffic and decides whether to forward or discard it. This decision is based on MAC address and forwarding table. the forwarding table is used by bridge while making filtering decisions.

Bridge know the physical address (MAC address) of all stations connected to it by listening network. Most of the bridges have capabilities to listen network automatically. Some other bridges don't have automatic learning capability, so in that case user or administrator has to manually add MAC address entries of connected hosts in bridging table for making decision.

Bridge checks incoming frame on a specific port connecting network segment, it then reads the physical address of destination host contained in the frame and the compares address with a forwarding table.

When it finds a match. It discovers segment to which destination station belongs and it needs to sends the frame only to that segment. So the bridge simply forward the frame on particular port to which the destination host connected to segment.

If source and destination both remain on same segment then bridge does not forward the frame. If source and destination are on different segment then bridge has to forward frame to segment to which destination is connected.

If the recipient’s MAC address is not in the list, then the  bridge blocks the data from passing. Bridge can connect different network architecture like token ring and Ethernet.

Bridge Forwarding Table

Physical Address	Port Number
1A-2B-3C-4D-5E-6F	1
4B-3D-7F-2C-6F-A6	2
4D-5E-6F1A-2B-3C	1
3C-4D-1A-2B-5E-6F	2

Forwarding table shows that the hosts that are connected to port number 1 and 2 of bridge. All the systems connected to port 1 are part of one  segment and systems connected to port 2 are part of another segment.

Types of Bridge

1. Simple Bridge
2. Multi port Bridge
3. Source route bridge
4. Translation Bridge
5. Local Bridge
6. Remote Bridge

1. Simple Bridge

Simple bridge are most primitive & less expensive bridge. Simple bridge cannot learn physical address automatically. The user has to enter the address manually in the forwarding table.

Whenever a new station is connected its physical address must be added to forwarding table. If station is disconnected or removed, then its address entry must be deleted from the forwarding table. So the table must be modified to have proper communication.

The simple bridge links only LAN segments.

2. Multi port Bridge

The bridge must have minimum two port in order to connect two segments. To connect multiple segments of LAN, the bridge should have more than 2 ports. The multi port bridge is the bridge is used to connect more than two segments.

3. Source route bridge

The source of each packet defines the bridge and LANs through which the packet should go before reaching destination. Frame contains different addresses like source, destination and bridge address. The station do majority of work here. Bridges do not filter broadcast packets and collisions are not transferred. Source route bridge avoids looping problem, it is designed for token ring

4. Translation Bridge

Translation bridge is used to connects network segment running at different speed or using different protocols.

5. Local Bridge

Local Bridge is used to connect network segments of same type and same location.

6. Remote Bridge

Remote bridge is designed to connect two network segments at distant location using WAN link.

Bridge advantages

Bridge is self configuring device.
It is used to interconnect two LANs.
It can minimize traffic on a segment.
It filter traffic and increases throughout on a network.
Primitive bridges often inexpensive.
It isolate a busy network from a not so busy one

Bridge disadvantages

It is slower than repeater and hub.
It is more expensive than repeater.
Does not scale to extremely large network.
It gives an error in bridging of different MAC protocol.
Does not limit the scope of broadcasts.